Process of preparing a chemically pre-formed (CPF) iron negative electrode with oxidizing gases

ABSTRACT

Provided is a process for preparing an electrode comprising an iron active material. The process comprises first fabricating an electrode comprising an iron active material, a polyvinyl alcohol binder and sulfur, and then treating the electrode with a gaseous oxidant to thereby create an oxidized surface. The resulting iron electrode is treated prior to any charge-discharge cycle to have the assessable surface of the iron active material in the same oxidation state as in discharged iron negative electrodes active material.

RELATED APPLICATIONS

The present application claims priority as a continuation application toU.S. application Ser. No. 14/478,990, filed Sep. 5, 2014; which claimspriority to provisional applications U.S. 61/874,177 filed on Sep. 5,2013 and U.S. 61/901,161 filed on Nov. 7, 2013, with the applicationsherein incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

Field of Invention

The present invention is in the technical field of energy storagedevices. More particularly, the present invention is in the technicalfield of rechargeable batteries using iron electrodes, and specificallyiron electrodes which have been chemically pre-formed (CPF) by a gaseousoxidant.

Related Art

Rechargeable batteries often require several charge-discharge cyclesprior to achieving optimum performance. During these early cycles,critical surface films are formed on the electrode surfaces that affectthe performance of the cell during later cycling. These early cycles arecommonly termed formation cycles in the battery industry. In the case ofnickel-iron batteries (Ni—Fe), 30 to 60 formation cycles are typicallyneeded to achieve the full capacity of the cell. Formation cyclingsometimes requires cycling at varied temperature regimes whichcomplicates the process. This formation process is expensive, timeconsuming, consumes electrolyte which needs replacing, and generates asignificant amount of gas. Therefore, reducing the number of formationcycles and simplifying the formation process is a worthy goal.

Manohar et. al. in “Understanding the factors affecting the formation ofCarbonyl Iron Electrodes in Rechargeable Alkaline Iron Batteries”, J.Electrochem. Soc., 159, 12, (2012) A 2148-2155, reported that one reasonfor the long formation time could also be the poor wettability of theiron electrode and the inaccessibility of the pores of the iron by theelectrolyte. As the pores became more accessible the charge anddischarge process produced a progressively rougher surface resulting inan increase in electrochemically active surface area and dischargecapacity. Triton X-100, a surfactant, reduced the number of cyclesrequired to achieve higher capacity presumably because it improvedaccess of the electrolyte to the pores.

U.S. Pat. No. 3,507,696 teaches that a mixture of FeO and Fe₂O₃ powdersfused with sulfur at 120° C. yields an active material that may be usedin an aqueous slurry to impregnate sintered nickel fiber plaques thatcan used as a negative electrode in a Ni—Fe battery. Several formationcycles are needed to achieve high capacity.

It would be of benefit to the industry to have an iron electrode whichis conditioned prior to any charge-discharge cycle so as to minimize theneed for formation cycles.

SUMMARY OF THE INVENTION

Provided is a process for preparing an electrode comprising an ironactive material, which comprises:

i) fabricating an electrode comprising an iron active material, and

ii) treating the surface of the electrode with a gaseous oxidant tothereby create an oxidized surface. In one embodiment, the oxidantcomprises ozone, chlorine or nitrous oxide.

In another embodiment, provided is an electrode which comprises an ironactive material, and which electrode has been preconditioned prior toany charge-discharge cycle to have the accessible surface of the ironmaterial in the same oxidation state as discharged iron negativeelectrode active material. The electrode is so preconditioned bytreating the electrode with a gaseous oxidant. In one embodiment, theoxidation state of the conditioned iron active material is +2, +2/+3, +3or +4.

Among other factors, the present invention provides a process andresulting iron electrode which addresses the mismatch in thestate-of-charge (SOC) of the anode and cathode that is present duringNi—Fe cell assembly. Use of the present process to pre-conditioned theiron electrode decreases the number of cycles, and time to achieve cellformation, electrolyte consumption, hydrogen gas generated, and theamount of water needed to refill the cell. In general, the process ofthe present invention leads to improved iron utilization in the cell.

BRIEF DESCRIPTION OF THE FIGURE OF THE DRAWING

FIG. 1 is an illustration of the interparticle contact between activematerial particles and the space between particles or pores that can befilled with oxidizing gas to precondition the surface of the electrodeparticles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Provided by the present invention is a chemically preconditioned ironelectrode and a method for its preparation. The present inventionchemically treats an iron metal electrode after the electrode isassembled to provide a preconditioned iron electrode. It is expectedthat the process of the present invention is amenable to a continuousprocess, and is therefore simpler and of lower cost than existentprocesses.

The preconditioned electrode may be prepared from a standard ironelectrode used in Ni—Fe cells. These iron electrodes can be comprised ofiron particles or mixtures thereof with sulfur, nickel, or other metalpowders, bonded to a substrate. In one embodiment, a conductive additivefor the iron electrode comprises nickel, carbon black or copper. In oneembodiment, an additive of the iron electrode comprises sulfur. Inanother embodiment, the coating of active material of the iron electrodecomprises a binder for the iron or iron active material, and additives.The binder is generally a polymer such as PVA, or a rubber. The use of aPVA binder has been found to be quite beneficial and advantageous.

In one embodiment, the iron electrode comprises about 50-90 wt % ironpowder, and in another embodiment from about 75-85 wt % iron powder;from about 5-30 wt % nickel powder, and in another embodiment from about12-20 wt % nickel powder; from 0.5-5.0 wt % binder, and in anotherembodiment from about 2.0-5.0 wt % binder; and, from 0.25-2.0 wt %sulfur, and in another embodiment, from about 0.25-1.0 wt % sulfur. Inone embodiment, the iron electrode comprises about 80 wt % iron powder,about 16 wt % nickel powder, about 3.5 wt % binder and about 0.5 wt %sulfur powder.

In one embodiment, the iron electrode can comprise additionalconventional additives, such as pore formers. In general, the porosityof the iron electrode is in the range of from 15-50%, and in oneembodiment from 35-45%.

The substrate used in the electrode can be comprised of a conductivematerial such as carbon or metal. The substrate for the iron electrodeis generally a single layer of a conductive substrate coated on at leastone side with a coating comprising the iron active material. Both sidesof the substrate can be coated. In one embodiment, the coating on atleast one side comprises iron and additives comprised of sulfur,antimony, selenium, tellurium, nickel, bismuth, tin, or a mixturethereof. The substrate is generally a metal foil, metal sheet, metalfoam, metal mesh, woven metal or expanded metal. In one embodiment, thesubstrate for the iron electrode is comprised of a nickel plated steel.It is generally of porous construction such as that provided by a mesh,or grid of fibrous strands, or a perforated metal sheet. The ironelectrode can also be sintered.

The iron electrodes of the present invention are chemicallypreconditioned with gaseous oxidants that are able to oxidize the ironsurface. These materials include but are not limited to: ozone,chlorine, or nitrous oxide. Gaseous oxidizing materials that arenon-toxic and volatile and yield reduction or thermal decompositionproducts that are also non-toxic and volatile are preferred. Preferredoxidants are ozone and nitrous oxide. The electrode is preconditioned byexposing the electrode to the oxidizing gas. This can be accomplished ina closed environment in which the gases released to contact theelectrode for the necessary period of time to oxidize the surface of theelectrode. The electrode may be rinsed with water after preconditioningwith the oxidant to remove the reduced form of the oxidant such aschloride. After rinsing, the electrode is then dried.

The length of time the electrodes are treated with the oxidizing gas canvary, but is generally until oxidation of the iron on the accessiblesurface of the electrode is observed. The temperature at which thetreatment is made is generally ambient, but it can be at highertemperatures. After the treatment, the electrode can be dried, ifneeded. It can be air dried or in an oven, for example. This is to makesure all of the oxidizing agent is removed.

In one embodiment, the treatment of the iron electrode is continueduntil the accessible surface of the iron material of the electrode is inthe same oxidation state as the electrode would in the discharged state.This is achieved by the oxidation treatment and can be determined usingconventional methods available.

While not wishing to be bound by theory, it is believed that nickel-ironbatteries may sometimes be assembled with the nickel cathode (positiveelectrode) in its discharged state and the iron anode (negativeelectrode) in its charged state. Thus, when the cell is assembled, thereis a mismatch between the state-of-charge (SOC) between the anode andcathode which is corrected during the formation process. During theformation process, it is believed that the low capacity of the earlycycles is due to the limited amount of discharge products (ie. Fe(OH)₂,Fe(OH)₃, and Fe₃O₄ depending on depth of discharge) that are formeduntil the proper conductivity, texture, and porosity of the ironelectrode is achieved. Consequently, the negative electrode is in ahigher SOC than the positive electrode for most of the formationprocess.

During the charge of a Ni—Fe cell there are typically two processes thatoccur at the anode surface, which are shown in Equations 1 and 2 below.Equation 1 is the desired conversion of discharge product, Fe(OH)₂, toiron metal. Equation 2 is the reduction of water to hydroxide andhydrogen gas. The two processes have very similar electrochemicalpotentials and both are usually active during the charge process.Fe(OH)₂+2e ⁻→Fe+2OH⁻ E°=−0.877 V  12H₂O+2e ⁻→H₂+2OH⁻ E°=−0.828V  2

However, when the negative electrode is at high SOC as in formation, thereaction in Equation 2 is more dominant since there is too littleFe(OH)₂ or other iron compounds with iron in its +2 or +3 oxidationstate to accept current from the cathode. The reaction in Equation 2consumes the water in the electrolyte which needs to be replaced andgenerates significant amount of gas that can become trapped between theelectrodes, further hindering desired electrochemical reactions at theelectrode surfaces. Gas generation can cause loss of adhesion of theactive material to the electrode further damaging the electrode.

It is believed that chemically pretreating the electrode with gaseousoxidants converts areas of the electrode that are accessible by thealkaline electrolyte, including pores, to iron compounds where iron isin its +2 or +3 oxidation state that are capable of being reduced toiron metal when an electrochemical current is applied in a cell. Theproducts of the pretreating of the iron electrode may be the same as thedischarge products on the iron electrode, or may be different. With someoxidants, rinsing may be necessary to remove the reduced form of theoxidant and convert the iron salts to iron hydroxides and iron oxides.Following these treatments, the products may comprise independently oras a mixture: Fe(OH)₂, Fe(OH)₃, Fe₃O₄, Fe₂O₃, FeO, and other ironoxides. As a result, the mismatch in the SOC of the anode and cathodethat is present during Ni—Fe cell assembly is minimized, if not avoidedall together. Use of the present process to prepare the iron electrodethereby decreases the number of cycles and time to achieve cellformation, electrolyte consumption, hydrogen gas generated, and theamount of water needed to refill the cell.

FIG. 1 shows a diagram of an electrode that has been preconditioned. Theiron particle active material, 1, retains interparticle contact, 2, andelectrical contact between the active materials and the substrate, 3, ismaintained. The surface of the electrode and the pores, 4, are able tobe contacted by the oxidant for preconditioning. Areas where there isinterparticle contact are not oxidized. Because the oxidation productsare electrically insulating, it is an advantage of this invention thatthe areas where there is interparticle contact are not oxidized,maintaining a conductive network between particles.

The present example is provided to further illustrate the presentinvention. It is not meant to be limiting.

Example

As an example, if an aqueous slurry consisting of 80% iron, 16% nickel,and 0.5% sulfur powders with 3.5% polyvinyl alcohol binder were pastedonto a perforated nickel sheet and dried, an electrode would be formed.This electrode could then be chemically preconditioned by being exposed,e.g., in a chamber, with an oxidizing gas such as ozone, chlorine ornitrous oxide. A slight orange-brown color would probably be observed onthe surface of the electrode due to oxidation. The electrodes might berinsed with water following treatment with the oxidizing gas. If twosample electrodes were cut from this sheet and tabs were TIG welded tothe top uncoated area of the electrode, and two sample cells wereconstructed using these negative electrodes by placing the negativeelectrode between two commercial Histar sintered positive nickelhydroxide electrodes, and for comparison, were compared to two identicalcells were constructed from identical materials except that the negativeelectrodes were not chemically preconditioned, advantageous resultswould be expected. For example, if the test cells containing CPF ironnegative electrodes and the control cells were subjected to anaccelerate life test at 55° C. with the following charge regime:

-   -   Cycle 1 (@ Room Temp): Charge: 1.0 A×1.5 hrs        -   Rest: 30 Min        -   Discharge: 0.1 A to 1.0 V        -   Rest: 30 Min    -   Cycle 2-100 (@ 55° C.): Charge: 1.0 A×1.5 hrs        -   Rest: 30 Min        -   Discharge: 0.1 A to 1.0 V        -   Rest: 30 Min

It is believed that if cells prepared with pre-conditioned ironelectrodes were compared to cells with negative electrodes that were notpreconditioned, a great advantage in capacity after only a few cycleswould be realized. Furthermore, the overall capacity for cells withpreconditioned electrodes is believed 17-19% higher for the life of thecell after formation. This is a further advantage of this invention.

While the foregoing written description of the invention enables one ofordinary skill to make and use what is considered presently to be thebest mode thereof, those of ordinary skill will understand andappreciate the existence of variations, combination, and equivalents ofthe specific embodiment, method, and examples therein. The inventionshould therefore not be limited by the above described embodiment,method and examples, but by all embodiments and methods within the scopeand spirit of the inventions and the claims appended therein.

What is claimed is:
 1. A process for preparing an electrode comprisingan iron active material, a polyvinyl alcohol binder and sulfur, whichcomprises: i) fabricating an electrode comprising an iron activematerial, a polyvinyl alcohol binder and sulfur, and ii) treating theelectrode with a gaseous oxidant to thereby create an oxidized surfaceprior to any charge-discharge cycle.
 2. The process of claim 1, thereinthe oxidant comprises ozone, chlorine, or nitrous oxide.
 3. The processof claim 2, wherein the oxidant comprises ozone.
 4. The process of claim1, wherein the electrode is treated prior to any charge-discharge cycleto have the accessible surface of the iron material in the sameoxidation state as discharged iron negative electrode active material.5. The process of claim 4, wherein the oxidation state of the treatediron active material is +2, +2/+3, +3 or +4.
 6. The process of claim 1,wherein the electrode further comprises a conductive additive.
 7. Theprocess of claim 6, wherein the conductive additive comprises nickel,copper or carbon black.
 8. The process of claim 1, wherein the electrodecomprises an iron active material comprising about: 50-90 wt % ironpowder 5-30 wt % nickel powder 0.5-5.0 w % polyvinyl alcohol binder, and0.25-20 w % sulfur.
 9. The process of claim 1, wherein the electrodecomprises a single layer of conductive substrate coated on at least oneside with a coating comprising the iron active material, polyvinylalcohol binder and sulfur.
 10. The process of claim 9, wherein thesubstrate is comprised of nickel plated steel.
 11. The process of claim1, wherein the porosity of the electrode is in the range of about15-50%.